Personal display with vision tracking

ABSTRACT

A display apparatus includes an image source, an eye position detector, and a combiner, that are aligned to a user&#39;s eye. The eye position detector monitors light reflected from the user&#39;s eye to identify the pupil position. If light from the image source becomes misaligned with respect to the pupil, a physical positioning mechanism adjusts the relative positions of the image source and the beam combiner so that light from the image source is translated relative to the pupil, thereby realigning the display to the pupil. In one embodiment, the positioner is a piezoelectric positioner and in other embodiments, the positioner is a servomechanism or a shape memory alloy.

TECHNICAL FIELD

The present invention relates to displays and, more particularly, todisplays that produce images responsive to a viewer's eye orientation.

BACKGROUND OF THE INVENTION

A variety of techniques are available for providing visual displays ofgraphical or video images to a user. For example, cathode ray tube typedisplays (CRTs), such as televisions and computer monitors are verycommon. Such devices suffer from several limitations. For example, CRTsare bulky and consume substantial amounts of power, making themundesirable for portable or head-mounted applications.

Flat panel displays, such as liquid crystal displays and field emissiondisplays, may be less bulky and consume less power. However, typicalflat panel displays utilize screens that are several inches across. Suchscreens have limited use in head mounted applications or in applicationswhere the display is intended to occupy only a small portion of a user'sfield of view.

More recently, very small displays have been developed for partial oraugmented view applications. In such applications, a portion of thedisplay is positioned in the user's field of view and presents an imagethat occupies a region 42 of the user's field of view 44, as shown inFIG. 1. The user can thus see both a displayed image 46 and backgroundinformation 48.

One difficulty with such displays is that, as the user's eye moves toview various regions of the background information, the user's field ofview shifts. As the field of view shifts, the position of the region 42changes relative to the field of view 44. This shifting may be desirablewhere the region 42 is intended to be fixed relative to the backgroundinformation 48. However, this shifting can be undesirable inapplications where the image is intended to be at a fixed location inthe user's field of view. Even if the image is intended to move withinthe field of view, the optics of the displaying apparatus may notprovide an adequate image at all locations or orientations of the user'spupil relative to the optics.

One example of a small display is a scanned display such as thatdescribed in U.S. Pat. No. 5,467,104 of Furness et. al., entitledVIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. Inscanned displays, a scanner, such as a scanning mirror or acousto-opticscanner, scans a modulated light beam onto a viewer's retina. Thescanned light enters the eye through the viewer's pupil and is imagedonto the retina by the cornea and eye lens. As will now be describedwith reference to FIG. 2, such displays may have difficulty when theviewer's eye moves.

As shown in FIG. 2, a scanned display 50 is positioned for viewing by aviewer's eye 52. The display 50 includes four principal portions, eachof which will be described in greater detail below. First, controlelectronics 54 provide electrical signals that control operation of thedisplay 50 in response to an image signal V_(IM) from an image source56, such as a computer, television receiver, videocassette player, orsimilar device.

The second portion of the display 50 is a light source 57 that outputs amodulated light beam 53 having a modulation corresponding to informationin the image signal V_(IM). The light source may be a directly modulatedlight emitter such as a light emitting diode (LED) or may be include acontinuous light emitter indirectly modulated by an external modulator,such as an acousto-optic modulator.

The third portion of the display 50 is a scanning assembly 58 that scansthe modulated beam 53 of the light source 57 through a two-dimensionalscanning pattern, such as a raster pattern. One example of such ascanning assembly is a mechanically resonant scanner, such as thatdescribed U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATUREOPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporatedherein by reference. However, other scanning assemblies, such asacousto-optic scanners may be used in such displays.

Optics 60 form the fourth portion of the display 50. The imaging optics60 in the embodiment of FIG. 2 include a pair of lenses 62 and 64 thatshape and focus the scanned beam 53 appropriately for viewing by the eye52. The scanned beam 53 enters the eye 52 through a pupil 65 and strikesthe retina 59. When scanned modulated light strikes the retina 59, theviewer perceives the image.

As shown in FIG. 3, the display 50 may have difficulty when the viewerlooks off-axis. When the viewer's eye 52 rotates, the viewer's pupil 65moves from its central position. In the rotated position all or aportion of the scanned beam 53 from the imaging optics 56 may not enterthe pupil 65. Consequently, the viewer's retina 59 does not receive allof the scanned light. The viewer thus does not perceive the entireimage.

One approach to this problem described employs an optics that expand thecross-sectional area of the scanned effective beam. A portion of theexpanded beam strikes the pupil 65 and is visible to the viewer. Whilesuch an approach can improve the effective viewing angle and help toensure that the viewer perceives the scanned image, the intensity oflight received by the viewer is reduced as the square of the beamradius.

SUMMARY OF THE INVENTION

A display apparatus tracks the orientation or position of a user's eyeand actively adjusts the position or orientation of an image source ormanipulates an intermediate component to insure that light enters theuser's pupil or to control the perceived location of a virtual image inthe user's field of view. In one embodiment, the display includes a beamcombiner that receives light from a background and light from the imagesource. The combined light from the combiner is received through theuser's pupil and strikes the retina. The user perceives an image that isa combination of the virtual image and the background.

In addition to the light from the background and light from the imagesource, additional light strikes the user's eye. The additional lightmay be a portion of the light provided by the image source or may beprovided by a separate light source. The additional light is preferablyaligned with light from the beam combiner. Where the additional lightcomes from a source other than the image source, the additional light ispreferably at a wavelength that is not visible.

A portion of the additional light is reflected or scattered by theuser's eye and the reflected or scattered portion depends in part uponwhether the additional light enters the eye through the pupil or whetherthe additional light strikes the remaining area of the eye. Thereflected or scattered light is then indicative of alignment of theadditional light to the user's pupil.

In one embodiment, an image field of a detector is aligned with thelight exiting the beam combiner. The detector receives the reflectedportion of the additional light and provides an electrical signalindicative of the amount of reflected light to a position controller.

In one embodiment, the detector is a low-resolution CCD array and theposition controller includes an electronic controller and a look uptable in a memory that provides adjustment data in response to thesignals from the detector. Data from the look up table drives apiezoelectric positioning mechanism that is physically coupled to asubstrate carrying both the detector and the image source.

When the detector indicates a shift in location of the reflectedadditional light, the controller accesses the look up table to retrievepositioning data. In response to the retrieved data, the piezoelectricpositioning mechanism shifts the substrate to realign the image sourceand the detector to the pupil.

In another embodiment, the CCD array is replaced by a quadrant-typedetector, including a plurality of spaced-apart detectors. The outputsof the detectors drive a control circuit that implements a searchfunction to align the scanned beam to the pupil.

In one embodiment, imaging optics having a magnification greater thanone helps to direct light from the image source and additional light tothe user's eye. Physical movement of the image source and detectorcauses an even greater movement of the location at which light from theimage source strikes the eye. Thus, small movements induced by thepiezoelectric positioning mechanism can track larger movements of thepupil position.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic representation of a combined image perceived bya user resulting from the combination of light from an image source andlight from a background.

FIG. 2 is a diagrammatic representation of a scanner and a user's eyeshowing alignment of a scanned beam with the user's pupil.

FIG. 3 is a diagrammatic representation of a scanner and a user's eyeshowing misalignment of the scanned beam with the user's pupil.

FIG. 4 is a diagrammatic representation of a display according to oneembodiment of the invention including a positioning beam and detector.

FIG. 5 is an isometric view of a head-mounted scanner including atether.

FIG. 6 is a diagrammatic representation of the display of FIG. 4 showingdisplacement of the eye relative to the beam position and correspondingreflection of the positioning beam.

FIG. 7A is a diagrammatic representation of reflected light striking thedetector in the position of FIG. 4.

FIG. 7B is a diagrammatic representation of reflected light striking thedetector in the position of FIG. 6.

FIG. 8 is a diagrammatic representation of the display of FIG. 2 showingthe image source and positioning beam source adjusted to correct themisalignment of FIG. 6.

FIG. 9 is a detail view of a portion of a display showing shape memoryalloy-based positioners coupled to the substrate.

FIG. 10 is a schematic of a scanning system suitable for use as theimage source in the display of FIG. 4.

FIG. 11 is a top plan view of a position detector including fourseparate optical detectors.

FIGS. 12A-C are diagrammatic representations of a display utilizing asingle reflective optic and a moving optical source.

FIG. 13 is a top plan view of a bi-axial MEMS scanner for use in thedisplay of FIG. 2.

FIG. 14 is a diagram of an alternative embodiment of a display includingan exit pupil expander and a moving light emitter.

FIG. 15A is a diagrammatic representative of nine exit pupils centeredover an eye pupil.

FIG. 15B is a diagrammatic representation of shifting of the eye pupilof FIG. 15A and corresponding shifting of the exit pupil array.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 4, a virtual retinal display 70 according to theinvention includes control electronics 72, a light source 74, a scanningassembly 58, and imaging optics 78. As with the embodiment of FIG. 2,the light source may be directly or indirectly modulated and the imagingoptics 78 are formed from curved, partially transmissive mirrors 62, 64that combine light received from a background 80 with light from thescanning assembly 58 to produce a combined input to the viewer's eye 52.The light source 74 emits light modulated according to image signalsV_(IM) the image signal source 56, such as a television receiver,computer, CD-ROM player, videocassette player, or any similar device.The light source 74 may utilize coherent light emitters, such as laserdiodes or microlasers, or may use noncoherent sources such as lightemitting diodes. Also, the light source 74 may be directly modulated oran external modulator, such as an acousto-optic modulator, may be used.One skilled in the art will recognize that a variety of other imagesources, such as LCD panels and field emission displays, may also beused. However, such image sources are usually not preferred because theytypically are larger and bulkier than the image source described in thepreferred embodiment. Their large mass makes them more difficult toreposition quickly as described below with reference to FIGS. 6-8.Moreover, although the background 80 is presented herein as a“real-world” background, the background light may be occluded or may beproduced by another light source of the same or different type.

Although the elements here are presented diagrammatically, one skilledin the art will recognize that the components are typically sized andconfigured for mounting to a helmet or similar frame as a head-mounteddisplay 67, as shown in FIG. 5. In this embodiment, a first portion 71of the display 67 is mounted to a head-borne frame 73 and a secondportion 75 is carried separately, for example in a hip belt. Theportions 71, 75 are linked by a fiber optic and electronic tether 77that carries optical and electronic signals from the second portion tothe first portion. An example of a fiber coupled scanner display isfound in U.S. Pat. No. 5,596,339 of Furness et. al., entitled VIRTUALRETINAL DISPLAY WITH FIBER OPTIC POINT SOURCE which is incorporatedherein by reference. One skilled in the art will recognize that, in manyapplications, the light source may be coupled directly to the scanningassembly 58 so that the fiber can be eliminated.

Returning to the display 70 of FIG. 4, the user's eye 52 is typically ina substantially fixed location relative to the imaging optics 78 becausethe display 70 is typically head mounted. For clarity, this descriptiontherefore does not discuss head movement in describing operation of thedisplay 70. One skilled in the art will recognize that the display 70may be used in other than head-mounted applications, such as where thedisplay 70 forms a fixed viewing apparatus having an eyecup againstwhich the user's eye socket is pressed. Also, the user's head may befree for relative movement in some applications. In such applications, aknown head tracking system may track the user's head position for coarsepositioning.

Imaging optics 78 redirect and magnify scanned light from the scanningassembly 58 toward the user's eye 52, where the light passes through thepupil 65 and strikes the retina 59 to produce a virtual image. At thesame time, light from the background 80 passes through the mirrors 62,64 and pupil 65 to the user's retina 59 to produce a “real” image.Because the user's retina 59 receives light from both the scanned beamand the background 80, the user perceives a combined image with thevirtual image appearing transparent, as shown in FIG. 1. To ease theuser's acquisition of light from partially or fully reflective mirrors62, 64, the imaging optics 78 may also include an exit pupil expanderthat increases the effective numerical aperture of the beam of scannedlight. The exit pupil expander is omitted from the Figures for clarityof presentation of the beam 53.

In addition to light from the light source 74, the imaging optics 78also receive a locator beam 90 from an infrared light source 92 carriedby a common substrate 85 with the light source 74. Though the locatorbeam 90 is shown as following a different optical path for clarity ofpresentation, the infrared light source 92 is actually positionedadjacent to the light source 74 so that light from the light source 74and light from the infrared light source 92 are substantially collinear.Thus, the output of the imaging optics 78 includes light from theinfrared light source 92. One skilled in the art will recognize that,although the infrared light source 92 and the light source 74 are shownas being physically adjacent, other implementations are easilyrealizable. For example, the infrared light source 92 may be physicallyseparated from the light source 74 by superimposing the locator beam 90onto the light from the light source 74 with a beam splitter andsteering optics.

Tracking of the eye position will now be described with reference toFIGS. 6-9. As shown in FIG. 6, when the user's eye 52 moves, the pupil65 may become misaligned with light from the light source 74 andinfrared light source 92. All or a portion of the light from the lightsource 74 and infrared source 92 may no longer enter the pupil 65 or mayenter the pupil 65 at an orientation where the pupil 65 does not directthe light to the center of the retina 59. Instead, some of the lightfrom the sources 74, 92 strikes a non-pupil portion 96 of the eye. As isknown, the non-pupil portion 96 of the eye has a reflectance differentand typically higher than that of the pupil 65. Consequently, thenon-pupil portion 96 reflects light from the sources 74, 92 back towardthe imaging optics 78. The imaging optics 78 redirect the reflectedlight toward an optical detector 98 positioned on the substrate 85adjacent to the sources 74, 92. In this embodiment, the detector 98 is acommercially available CCD array that is sensitive to infrared light. Aswill be described below, in some applications, other types of detectorsmay be desirable.

As shown in FIG. 7A, when the user's eye is positioned so that lightfrom the sources 74, 92 enters the pupil (i.e., when the eye ispositioned as shown in FIG. 4), a central region 100 of the detector 98receives a low level of light from the imaging optics 78. The area oflow light resulting from the user's pupil will be referred to herein asthe pupil shadow 106. When the eye 52 shifts to the position shown inFIG. 6, the pupil shadow shifts relative to the detector 88 as shown inFIG. 7B.

The detector data, which are indicative of the position of the pupilshadow 106 are input to an electronic controller 108, such as amicroprocessor or application specific integrated circuit (ASIC).Responsive to the data, the controller 108 accesses a look up table in amemory device 110 to retrieve positioning data indicating an appropriatepositioning correction for the light source 74. The positioning data maybe determined empirically or may be calculated based upon known geometryof the eye 52 and the scanning assembly 58.

In response to the retrieved positioning data, the controller 110activates X and Y drivers 112, 114 to provide voltages to respectivepiezoelectric positioners 116, 118 coupled to the substrate 85. As isknown, piezoelectric materials deform in the presence of electricalfields, thereby converting voltages to physical movement. Therefore, theapplied voltages from the respective drivers 112, 114 cause thepiezoelectric positioners 116, 118 to move the sources 74, 92, asindicated by the arrow 120 and arrowhead 122 in FIG. 8.

As shown in FIG. 8, shifting the positions of the sources 74, 92 shiftsthe locations at which light from the sources 74, 92 strikes the user'seye, so that the light once again enters the pupil. The pupil shadow 106once again returns to the position shown in FIG. 7A. One skilled in theart will recognize that the deformation of the piezoelectric positioner116 is exaggerated in FIG. 8 for demonstrative purposes. However,because the mirrors 62, 64 have a magnification greater than one, smallshifts in the position of the substrate 85 can produce larger shifts inthe location at which the light from the light source 74 arrives at theeye. Thus, the piezoelectric positioners 112, 114 can produce sufficientbeam translation for many positions of the eye. Where even larger beamtranslations are desirable, a variety of other types of positioners,such as electronic servomechanisms may be used in place of thepiezoelectric positioners 112, 114. Alternatively, shape memoryalloy-based positioners 113, such as equiatomic nickel-titanium alloys,can be used to reposition the substrate as shown in FIG. 9. Thepositioners 113 may be spirally located, as shown in FIG. 9 or may be inany other appropriate configuration. One skilled in the art will alsorecognize that the imaging optics 78 does not always requiremagnification, particularly where the positioners 116, 118 are formedfrom a mechanism that provides relatively large translation of thescanner 70.

FIG. 10 shows one embodiment of a mechanically resonant scanner 200suitable for use as the scanning assembly 58. The resonant scanner 200includes as the principal horizontal scanning element, a horizontalscanner 201 that includes a moving mirror 202 mounted to a spring plate204. The dimensions of the mirror 202 and spring plate 204 and thematerial properties of the spring plate 204 are selected so that themirror 202 and spring plate 204 have a natural oscillatory frequency onthe order of 1-100 kHz. A ferromagnetic material mounted with the mirror202 is driven by a pair of electromagnetic coils 206, 208 to providemotive force to mirror 202, thereby initiating and sustainingoscillation. Drive electronics 218 provide electrical signal to activatethe coils 206, 208.

Vertical scanning is provided by a vertical scanner 220 structured verysimilarly to the horizontal scanner 201. Like the horizontal scanner201, the vertical scanner 220 includes a mirror 222 driven by a pair ofcoils 224, 226 in response to electrical signals from the driveelectronics 218. However, because the rate of oscillation is much lowerfor vertical scanning, the vertical scanner 220 is typically notresonant. The mirror 222 receives light from the horizontal scanner 201and produces vertical deflection at about 30-100 Hz. Advantageously, thelower frequency allows the mirror 222 to be significantly larger thanthe mirror 202, thereby reducing constraints on the positioning of thevertical scanner 220.

In operation, the light source 74, driven by the image source 56 (FIG.8) outputs a beam of light that is modulated according to the imagesignal. At the same time, the drive electronics 218 activate the coils206, 208, 224, 226 to oscillate the mirrors 202, 222. The modulated beamof light strikes the oscillating horizontal mirror 202, and is deflectedhorizontally by an angle corresponding to the instantaneous angle of themirror 202. The deflected light then strikes the vertical mirror 222 andis deflected at a vertical angle corresponding to the instantaneousangle of the vertical mirror 222. The modulation of the optical beam issynchronized with the horizontal and vertical scans so that at eachposition of the mirrors, the beam color and intensity correspond to adesired virtual image. The beam therefore “draws” the virtual imagedirectly upon the user's retina. One skilled in the art will recognizethat several components of the scanner 200 have been omitted for clarityof presentation. For example, the vertical and horizontal scanners 201,220 are typically mounted in fixed relative positions to a frame.Additionally, the scanner 200 typically includes one or more turningmirrors that direct the beam such that the beam strikes each of themirrors a plurality of times to increase the angular range of scanning.

FIG. 11 shows one realization of the position detector 88 in which theCCD array is replaced with four detectors 88A-88D each aligned to arespective quadrant of the virtual image. When the user's eye 52 becomesmisaligned with the virtual image, the pupil shadow 106 shifts, asrepresented by the broken lines in FIG. 10. In this position, theintensity of light received by one or more of the detectors 88A-88Dfalls. The voltage on the positioners 116, 118 can then be varied torealign the scanned light to the user's eye 52. Advantageously, in thisembodiment, the outputs of the four quadrant detector can form errorsignals that, when amplified appropriately, may drive the respectivepositioners 114, 116 to reposition the light emitter 74.

A further aspect of the embodiment of the display 70 of FIG. 8 is z-axisadjustment provided by a third positioner 128 that controls the positionof the light source 74 and scanner 76 along a third axis. The thirdpositioner 128, like the X and Y positioners 114, 116 is a piezoelectricpositioner controlled by the electronic controller 108 through acorresponding driver 130.

As can be seen from FIG. 8, when the user's eye 52 rotates to view anobject off-axis and the X and Y positioners 116, 118 adjust the positionof the light source 74, the distance between the scanner 76 and thefirst mirror 64 changes slightly, as does the distance between the firstmirror 64 and the eye 52. Consequently, the image plane defined by thescanned beam may shift away from the desired location and the perceivedimage may become distorted. Such shifting may also produce an effectiveastigmatism in biocular or binocular systems due to difference in thevariations between the left and right eye subsystems. To compensate forthe shift in relative positions, the controller 108, responsive topositioning data from the memory 110, activates the third positioner130, thereby adjusting the z-axis position of the light source 74. Theappropriate positioning data can be determined empirically or may bedeveloped analytically through optical modeling.

One skilled in the art will also recognize that the controller 108 canalso adjust focus of the scanned beam 53 through the third positioner130. Adjustment of the focus allows the controller to compensate forshifts in the relative positions of the scanning assembly 76, mirrors62, 64 and eye 52 which may result from movement of the eye, temperaturechanges, pressure changes, or other effects. Also, the controller 108can adjust the z-axis position to adapt a head-mounted display todifferent users.

Although the embodiments herein are described as having positioningalong three orthogonal axes, the invention is not so limited. First,physical positioning may be applied to other degrees of motion. Forexample, rotational positioners may rotate the mirrors 62, 64, the lightsource 74 or the substitute 85 about various axes to provide rotationalpositioning control. Such an embodiment allows the controller log toestablish position of the virtual image (e.g. the region 42 of FIG. 1).By controlling the position of the virtual image, the controller 108 canmove the region 42 to track changes in the user's field of view. Theregion 42 can thus remain in a substantially fixed position in theuser's field of view. In addition to rotational freedom, one skilled inthe art will recognize that the three axes are not limited to orthogonalaxes.

While the embodiments described herein have included two mirrors 62, 64,one skilled in the art will recognize that more complex or less complexoptical structures may be desirable for some applications. For example,as shown in FIGS. 12A-C, a single reflective optics 300 can be used toreflect light toward the viewer's eye 52. By tracing the optical paths302 from the scanning assembly 58 to the pupil 65, the correspondingposition and angular orientation of the scanning assembly 58 can bedetermined for each eye position, as shown in FIGS. 12A-C.

The determined position and orientation are then stored digitally andretrieved in response to detected eye position. The scanning assembly 58is then moved to the retrieved eye position and orientation. Forexample, as shown in FIG. 12B, when the field of view of the eyes iscentered, the scanning assembly 58 is centered. When the field of viewis shifted left, as shown in FIG. 12A, the scanning assembly 58 isshifted right to compensate.

To reduce the size and weight to be moved in response to the detectedeye position, it is desirable to reduce the size and weight of thescanning assembly 58. One approach to reducing the size and weight is toreplace the mechanical resonant scanners 200, 220 with amicroelectromechanical (MEMS) scanner, such as that described in U.S.Pat. No. 5,629,790 entitled MICROMACHINED TORSIONAL SCANNER toNeukermans et. al. and U.S. Pat. No. 5,648,618 entitled MICROMACHINEDHINGE HAVING AN INTEGRAL TORSION SENSOR to Neukermans et. al., each ofwhich is incorporated herein by reference. As described therein andshown in FIG. 13, a bi-axial scanner 400 is formed in a siliconsubstrate 402. The bi-axial scanner 400 includes a mirror 404 supportedby opposed flexures 406 that link the mirror 404 to a pivotable support408. The flexures 406 are dimensioned to twist torsionally therebyallowing the mirror 404 to pivot about an axis defined by the flexures406, relative to the support 408. In one embodiment, pivoting of themirror 404 defines horizontal scans of the scanner 400.

A second pair of opposed flexures 412 couple the support 408 to thesubstrate 402. The flexures 412 are dimensioned to flex torsionally,thereby allowing the support 408 to pivot relative to the substrate 402.Preferably, the mass and dimensions of the mirror 404, support 408 andflexures 406, 412 are selected such that the mirror 404 resonates, at10-40 kHz horizontally with a high Q and such that the support 408pivots at frequencies that are preferably higher than 60 Hz, although insome applications, a lower frequency may be desirable. For example,where a plurality of beams are used, vertical frequencies of 10 Hz orlower may be acceptable.

In a preferred embodiment, the mirror 404 is pivoted by applying anelectric field between a plate 414 on the mirror 404 and a conductor ona base (not shown). This approach is termed capacitive drive, because ofthe plate 414 acts as one plate of a capacitor and the conductor in thebase acts as a second plate. As the voltage between plates increases,the electric field exerts a force on the mirror 404 causing the mirror404 to pivot about the flexures 406. By periodically varying the voltageapplied to the plates, the mirror 404 can be made to scan periodically.Preferably, the voltage is varied at the mechanically resonant frequencyof the mirror 404 so that the mirror 404 will oscillate with littlepower consumption.

The support 408 may be pivoted magnetically or capacitively dependingupon the requirements of a particular application. Preferably, thesupport 408 and flexures 412 are dimensioned so that the support 408 canrespond frequencies well above a desired refresh rate, such as 60 Hz.

An alternative embodiment according to the invention, shown in FIG. 14includes a diffractive exit pupil expander 450 positioned between thescanning assembly 58 and the eye 52. As described in U.S. Pat. No.5,701,132 entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL toKollin et. al. which is incorporated herein by reference, at each scanposition 452, 454 the exit pupil expander 450 redirects the scanned beamto a plurality of common locations, to define a plurality of exit pupils456. For example, as shown in FIG. 15A, the exit pupil expander 450 mayproduce nine separate exit pupils 456. When the user's pupil 65 receivesone or more of the defined exit pupils 456, the user can view thedesired image.

If the user's eye moves, as shown in FIG. 15B, the pupil 65 still mayreceive light from one or more of the exit pupils 456. The user thuscontinues to perceive the image, even when the pupil 65 shifts relativeto the exit pupils 456. Nevertheless, the scanning assembly 58 (FIGS.12A-12C) shifts, as indicated by the arrows 458 in FIG. 14 and arrows460 in FIG. 15B to center the array of exit pupils 456 with the user'spupil 65. By re-centering the array relative to the pupil 65, the numberof exit pupils 456 can be reduced while preserving coupling to the pupil65.

Although the invention has been described herein by way of exemplaryembodiments, variations in the structures and methods described hereinmay be made without departing from the spirit and scope of theinvention. For example, the positioning of the various components mayalso be varied. In one example of repositioning, the detector 88 andinfrared source 92 may be mounted separately from the light source 74.In such an embodiment, the detector 98 and infrared source 92 may bemounted in a fixed location or may be driven by a separate set ofpositioners. Also, in some applications, it may be desirable toeliminate the infrared source 92. In such an embodiment, the detector 98would monitor reflected visible light originating from the light source74. Also, the infrared beam and scanned light beam may be made collinearthrough the use of conventional beam splitting techniques. In stillanother embodiment, the piezoelectric positioners 116, 118 may becoupled to the mirror 64 or to an intermediate lens 121 to produce a“virtual” movement of the light source 74. In this embodiment,translation of the mirror 64 or lens 121 will produce a shift in theapparent position of the light source 74 relative to the eye. Byshifting the position or effective focal length of the lens 121, thelens 121 also allows the display to vary the apparent distance from thescanner 200, 400 to the eye 52. For example, the lens 121 may be formedfrom or include an electro-optic material, such as quartz. The effectivefocal length can then be varied by varying the voltage across theelectro-optic material for each position of the scanner 200, 400.Moreover, although the horizontal scanners 200, 400 are described hereinas preferably being mechanically resonant at the scanning frequency, insome applications the scanner 200 may be non-resonant. For example,where the scanner 200 is used for “stroke” or “calligraphic” scanning, anon-resonant scanner would be preferred. One skilled in the art willrecognize that, although a single light source is described herein, theprinciples and structures described herein are applicable to displayshaving a plurality of light sources. In fact, the exit pupil expander450 of FIG. 14 effectively approximates the use of several lightsources. Further, although the exemplary embodiment herein utilizes thepupil shadow to track gaze, a variety of other approaches may be withinthe scope of the invention, for example, reflective techniques, suchknown “glint” techniques as may be adapted for use with the describedembodiments according to the invention may image the fundus or featuresof the iris to track gaze. Accordingly, the invention is not limitedexcept as by the appended claims.

What is claimed is:
 1. A method of producing an image for viewing by aneye in an image field with a portable head mounted display, comprisingthe steps of: emitting light from a first location, modulating the lightin a pattern corresponding to the image; scanning the light through aperiodic scan pattern in two axes; wherein emitting light includesmodifying the periodic scan pattern with guiding optics to produce avisible image at the image field; producing a positioning beam;directing the positioning beam along a first path toward the eye;receiving a portion of light reflected from the eye with an opticaldetector; producing an electrical signal responsive to the receivedreflected light; identifying a pupil position responsive to theelectrical signal; and physically repositioning the first locationangularly about a first axis in response to the electrical signal,wherein the angular repositioning of the first location about the firstaxis corresponds substantially to alignment of the periodic scan patternwith the identified pupil position; determining an optical path lengthchange corresponding to a change in optical path lengths produced byangular movement of the scanner or movement of the pupil relative to thescanner, the optical path extending from the scanner to the pupil; andrepositioning the first location to a revised location along a secondoptical axis relative to the optical element a distance corresponding tothe determined optical path length change, the revised locationestablishing the image field at the pupil location while the guidingoptics remains substantially stationary relative to the eye.
 2. Themethod of claim 1 wherein an image source produces the light and whereinthe step of physically repositioning the first location in response tothe electrical signal includes physically repositioning the image sourcerelative to the user's eye.
 3. The method of claim 2 wherein the step ofphysically repositioning the image source includes activating apiezoelectric positioner coupled to the image source.
 4. The method ofclaim 2 wherein the step of physically repositioning the image shownincludes activating a shape memory alloy coupled to the image source. 5.The method of claim 1 wherein the optical detector includes a detectorarray and wherein the step of producing an electrical signal responsiveto the received reflected light includes outputting data from thedetector array.
 6. The method of claim 1 wherein the positioning beam isan infrared beam.
 7. The method of claim 1 wherein the step of producingan electrical signal includes the steps of: outputting data from thedetector array; retrieving data stored in a memory; and producing theelectrical signal in response to the retrieved data.
 8. The method ofclaim 1 wherein a portion of the emitted light forms the positioningbeam.
 9. The method of claim 1 wherein the guiding optics include alens.
 10. The method of claim 9 wherein the guiding optics furtherinclude a turning reflector.